1. Field of the Invention
The present invention relates to nanocrystalline phosphors. The present invention also relates to a method for preparing such phosphors and devices which contain such phosphors.
2. Description of the Background
High performance cathodoluminescence (CL) phosphors capable of working at low beam voltages have been an objective for field emitter displays (FED). Since FEDs operate at low voltages ( less than 5 kV) under high vacuum ( less than 10xe2x88x927 Torr), it requires phosphors which emit enough light perceivable by human eyes ( greater than 300 cd/m2) and are stable under electron bombardment under high vacuum (low outgassing). A review of cathodoluminescence is provided in L. Ozawa, Cathodoluminescence, Theory and Applications, VCH Publishers, New York, 1990, which is incorporated herein by reference in its entirety.
Existing commercial phosphors, however, are powders with a grain size of a few micrometers which do not meet the requirements for effective use in FEDs. The power efficiency of a phosphor is largely dependent on its effectiveness of materials. In other words, a phosphor will be inefficient if its grains contain a xe2x80x9cdead portion.xe2x80x9d Although the causes of dead portions are yet to be clarified, lattice defects, de-activating impurities, and locally charged sites are thought to be responsible. The dead portion effect becomes especially significant in low voltage electron excitation (beam voltage  less than 1,000 V).
Most phosphors possess activators (or dopants) embedded in a wide band gap semiconductor matrix (or host). The physical mechanisms behind cathodoluminescence are various and remain, at least partly, controversial. Basically, the luminescence results from electron relaxation from an excited level to a ground level within an activator through a highly localized transition path, or from charge carrier recombination following an excitation-created charge separation (the formation of exciton). The latter mechanism is believed to be less localized and hence to be easily perturbed by impurities or other alien quenchers. The matrix crystals, serving as operating media, are required to provide xe2x80x9cgoodxe2x80x9d environment for the transitions to happen without dissipation. It has been thought that the presence of perfect crystals of the host would decrease the dead portion effect. Efforts have been, in turn, made toward making phosphor grains in the form of perfect crystals.
The processes adopted so far in phosphor fabrication are xe2x80x9cmixing-and-firingxe2x80x9d routes based on solid state reactions. The extent to which the reactions are completed is determined by two vital steps: the homogeneous distribution of the solid precursors and the extent of fusion. Since almost all the solid state reactions involved, such as Mnxe2x80x94Znxe2x80x94Si combination and Cexe2x80x94Yxe2x80x94Si combination, are slow, the quality of the end productsis governed by efficient and complete incorporation of the dopant at certain concentration levels used in the phosphors. Additionally, the firing-led fusion occurs only between the adjacent partners, and the precursor particles brought together during the xe2x80x9cmixing.xe2x80x9d Any inhomogeneity of the xe2x80x9cmixingxe2x80x9d results in inefficient matrix formation as well as nonuniform incorporation of the fluorescence centers (dopants) in the matrix. Inefficient fusion leads to the formation of a dead portion even after firing. The dead portion, in this context, includes xe2x80x9cdummy partsxe2x80x9d of the materials which contain no dopants and in turn yield no light output, and jammed parts of the materials which are overpopulated by the dopants and therefore detrimental to the output light by internal quenching.
Nanocrystals (usually with size of a few nanometers), grown primarily in a quick solution reaction, are defect-free and well-defined chemical entities. Efforts have been invested by Rameshwar N. Bhargava et al (U.S. Pat. No. 5,637,258) in preparing nanosized phosphors of doped-zinc sulfide and yttrium oxides. The resulting phosphors have, in fact, an efficiency which is much lower than for the micron sized samples.
Thus, there remains a need for phosphors which do not contain any dead portions and which exhibit a high efficiency. There also remains a need for a method for preparing such phosphors and for devices which contain such phosphors.
Accordingly, it is one object of the present invention to provide novel phosphors.
It is another object of the present invention to provide novel phosphors which contain few or no dead portions.
It is another object of the present invention to provide novel phosphors which exhibit high efficiency.
It is another object of the present invention to provide novel nanosized phosphors which contain few or no dead portions.
It is another object of the present invention to provide novel nanosized phosphors which exhibit a high efficiency.
It is another object of the present invention to provide novel methods for preparing such phosphors.
It is another object of the present invention to provide novel devices which contain such phosphors.
These and other objects, which will become apparent during the following detailed description, have been achieved by the inventors"" discovery that a method which involves:
(1) forming a solution or slurry which contains nano-sized particles of the phosphor precursors
(2) eliminating the liquid medium in the solution or slurry by heating to an appropriate temperature, preferably slightly above the boiling point of the medium, to obtain a solid residue; and
(3) firing the solid residue, provides phosphors which exhibit an improved efficiency.
In a particularly preferred embodiment of the present method, a first precursor which contains metal-ion-doped metal oxide particles having a greatest dimension of 500 nm or less, preferably 1 to 300 nm , more preferably 2 to 10 nm, in alcosol form is mixed with a second precursor, e.g., containing silica (SiO2) having a greatest dimension of 500 nm or less, preferably 1 to 300 nm , more preferably 2 to 10 nm and the resulting mixture is sonicated, followed by the calcination of the resulting gel.
Another preferred embodiment of the present method includes the following steps:
(1) forming a suspension, as the first precursor, of a metal-ion-doped or a metal-ion-attached nanosized particles, i.e., nanoparticles whose surfaces are linked, chemically or physically, to dopant metal ions;
(2) forming a suspension of a metal-ion-doped or metal-ion-attached nanosized silica particles;
(3) mixing the suspension of nano-sized particles of a first precursor with the suspension of nano-sized particles of silica, to obtain a homogeneous gel containing said first precursor and silica;
(4) sonicating the homogeneous gel containing the first precursor and silica, to obtain a sonicated gel;
(5) drying the homogeneous gel containing the first precursor and silica, to obtain a residue;
(6) adding a flux material to said residue, to obtain a mixed powder; and
(7) firing the mixed powder.